Traveling wave tube



Jan. 6, 1959 R. KOMPFNER 2,367,744

TRAVELING WAVE TUBE Filed Sept. so, 1953 A TTORNEV TRAVELING WAVE TUBE.

Rudolf Kompfner, Far Hills, N. J., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of NewYorkApplication September. 30, 1953, Serial No. 383,247

4 Claims. (Cl. 315-35) This. invention relates to improvements in radiofrequency devices which utilize the interaction between a travelingelectromagnetic Wave and an. electron beam to amplify theelectromagnetic wave, now generally designated as traveling wave tubes,and more particularly to improvements in the focusing of electron beamsinsuch devices.

Among the various. problems associated with theoperation of such tubesis that of keeping the electron beam in alignment with the wave guidingstructurev used for propagating the traveling electromagnetic wavewhereby a minimum number of electrons will be lost in. strikingthestructure while yet the electron flow can be kept close enough to thestructure for good interaction withtheradio frequency electric fieldsassociated therewith. To this end, it has been found advantageous toemploy magnetic focusing to confine the electron beam. In, United StatesPatent 2,847,607, issued August 12, 1958, of J. R. Pierce it isshownthat good magnetic focusing, can be achieved by a periodic successionof'magnetic regions of a spatially alternating axially symmetriclongitudinal magnetic field along the path of flow. Such periodicfocusing isfound to result in considerable economies in thesize and.weight of the auxiliary equipment necessary for magnetic focusing. In.particular, in the arrangements described in the above-identifiedpatent,the periodic spatially alternating magnetic field regions are achievedby spacing along the path. of flow a succession of annular members whichserve as pole pieces and poling successive pole pieces oppositely bybridging between each pair of, adjacent annular members, one of'asuccession. offpermanent magnets which are magnetized in a directionparallel to that of the electron flow.

With arrangements of this kind,' it is found. that with changes in themagnetic period and the magnetic field strength along the path of flow,the focusing exhibits pass and stop bands, i. e., regions of good andpoor focusing. The highest beam transmission efliciency usually can behad by'operation in the pass band corresponding to the shortest magneticperiod. Moreover, it isv found that the magnetic period and magneticfield strength necessary for optimum focusing in thispass-band isrelated to the beam current. and beam perveance. Accordingly, intraveling Wave tubes which use a large beam current and high'beamperveance, short magnetic periods and high magnetic field strengths areimportant for high beam. transmission efijciencies. However, in thevarious focusing arrangements described in the aforementioned patent, ashort magnetic period requires a short pole-piece to. pole-piece spacingand hence a short permanent magnet for bridging adjacent pairs of polepieces. Short permanent magnets generally have low efficiencies, and,accordingly, it becomes difficult to achieve. high magnetic fieldstrengths along the path of flow conveniently. Ac-

-cording ly,..it has been. difficult hitherto to utilize periodicfocusingprinciples eificiently for focusing electron beams of highdensity and high perveances. One object of, the present invention is tobetter to adapt the principles of nite ates atentO periodicfocusing tobeams of high density and perveances where short magnetic periods aredesirable.

To this end, the principal feature of the. present invention. is a.periodic focusing arrangement which comprises. a succession of annularmembers which serve as pole pieces along the path of flow and associatedtherewith a like succession, of annular permanent magnets which aremagnetized in a radial direction, successive magnets being magnetized ina reverse sense. These are enclosed bya. magnetically soft. cylindricalshield which simultaneously serves as. the return path of themagnetic-flux between adjacent permanentmagnets.

One advantage of this arrangement is that the com? binationof thecylindrical shield. andv the. pole pieces 1 servesto concentrate the.flux. from the permanent magnets along the path of flow causing fewstray fields and in, creasing the-efliciency of the permanent magnets.

Moreover, since the magnetomotive force developed across, the gapsbetween adjacent pole pieces is related to the length along the magneticaxis of the. permanent magnets, higher magnetomotive forces and highermagnetic field intensities can be achieved efiiciently by extending thedimension, of. the magnets in. the radial direction with no. change inthe periodicity of the pole pieces along the path of flow.

The invention'will be-better understood fromthe following more detaileddescription taken in. conjunction with theaccompanying drawing whichillustrates the embodiment of the principles of the. invention in avhelix-type traveling wave tube.

Referring more particularly to the drawing, in the illustrativetravelingwave tube 10. shown in longitudinal cross sectioman evacuatedglassenvelopell. houses the various tube, elements. At opposite. ends ofthe envelope an electron. source 12. and a target electrode 13 define alongitudinalpath of flow for the electronbeam along the axis of theelongated portion- 14 of the tube envelope. The electron source 12 is aconventional-type electron gun for providing a concentrated electronbeam. Disposed along the path of fiow coaxial withthe longitudinal axisof the tube is the Wave interaction circuit comprising the helicallywoundconductor 15. The helix is adapted to propagate a travelingwavewith an axial phase velocity sufliciently approximating the. velocity ofthe electron beam that substantial interaction results whereby thewaveis amplified. The electron velocity is adjusted by the beamIaccelerating voltage on the helix 15. The various lead-in connectionsfor applying the necessary D.-C. voltages to the various tube. elementshave been omitted for the sake of. simplicity.

Aninput wave to be amplified isv applied to the wave interaction.circuit by the external helix 16 which, .is coupledelectromagneticallyto the. circuit helix 15. The principles of coupledhelices are setforth in my United States Patent 2,811,673, issuedOctober 29, 1957. For good couplingin this way, the winding of the outerhelix 16 should be in a sense opposite to that of the inner helix 15 butthe pitch of the. outer helix should be approximately the same as thatof the inner helix. 15 whereby the axial phase velocities of wavespropagating therealong will be substantially similar for the twohelices. Additionally, the outer helix should overlap the inner helixfora distance whichis a quarter of the beat wavelength. This beatwavelength may be defined as follows: The wave propagating along one ofthe two coupled helices has a phase velocity only slightly differentfrom that of a wave propagating on one. of the helices when uncoupled,but the amplitude of the wave propagating along one of two coupledhelices varies sinusoidally with distance, exhibiting a phenomenon whichmay be cal'ledfspatial beating. Thelength ofone cycle of this,

phenomenon for conven'ieve is called the beat wave-s length. The end ofthe outer helix downstream along the path of flow (it is to be notedthat the terms upstream and downstream are used with reference torelative distancesrfrom the source rather than the inclination of thetube) is terminated to be substantially reflectionless by a resistivetermination 17, and the other end is connected to the inner conductor ofa coaxial line 18 by means of which energy is guided from a suitablesignal source to the input connection of the traveling wave tube.

The output wave is abstracted downstream from the interaction circuitfor use by utilization apparatus by a similar coupled helix arrangementcomprising the outer helix 19, its resistive termination 20, and theoutput coaxial line 21, corresponding, respectively, to the outer helix16, its resistive termination 17, and the input coaxial line 18.

To minimize various undesirable effects resulting from mismatches at thecoupling connections, it is advantageous to deposit on the inner surfaceof the elongated portion of the tube envelope 14 along regions adjacentthe uncoupled upstream and downstream end turns of the inner helix 16coatings 22 of a dissipative material such as aquadag.

For focusing the electron beam along its path of flow between theelectron source 12 and the target 13 in accordance with the principlesof the invention, a succession of annular members 30 of a magneticallysoft material, such as permalloy, ferrite, or soft iron, is disposedalong the path of flow. Successive members are spaced apart by a shortgap 31. These members 30 are adapted to serve as pole pieces forestablishing a spatially alternating time-constant longitudinal magneticfield along the path of flow. For polarizing successive members ppositely, there is associated with each successive annular member 30 adifferent one of a like succession of annular disk-shaped permanentmagnets 32, each of which is magnetized in a radial direction, thepolarities of successive permanent magnets being reversed in sense inthe manner illustrated whereby adjacent members 30 of the succession arein contact with oppositely poled faces of the permanent magnets 32. Amagnetically soft cylindrical shield 34 surrounds the succession ofpermanent magnets. Besides serving to confine the magnetic flux, theshield serves as the return path for the flux between adjacent permanentmagnets. The shield 34 should have a wall thickness suflicient to keepit from becoming magnetically saturated. As a consequence, there areformed along the path of flow a succession of magnetic loops in each ofwhich the only air (or unity permeability) gap is that corresponding tothe gap 31 between successive annular members 30. As a consequence,there will be developed in these gaps magnetic fields of highintensities, and the magnetic flux thereacross will permeate the path ofelectron flow. Because of the reversal in the polarities of successivepole pieces 30, the direction of the magnetic flux between successivegaps'32 reverses, and, accordingly,

the magnetic field along the path of flow will be spatially alternatinghaving a magnetic period which corresponds to twice the mean separationof successive pole pieces.

The available magnetomotive force for developing flux across each airgap is now proportional to approximately twice the radial dimension ofthe permanent magnets 32 whereas in the previously known arrangementswhere I permanent magnets are used to bridge successive pairs of polepieces, the magnetomotive force useful is proportional approximately toonly one half the permanent magnet length.

Additionally, where high magnetic field intensities along the path offlow are desired, high magnetomotive forces may be achieved byincreasing the radial extension of v the permanent magnets to dimensionslong compared with the mean separation of successive pole pieces.However, :in previously known arrangements of the kind described, 7 thelength of the magnet path in each magnetic path 4 l a is limited to themean separation of successive pole pieces, making difiicult therealization of high magnetomotive forces desired for creating high fluxdensities in the gaps between successive pole pieces and along the pathof flow.

Various other advantages stemming from this improved form of focusingstructure will be evident to workers skilled in the art.

It is to be understood that the specific embodiment which has beendescribed is merely illustrative of the principles of the invention,various arrangements may be devised by one skilled in the art withoutdeparting from the spirit and scope of the invention. In particular, inarrangements of this kind it is generally advantageous to have the polepieces as close to the path of electron flow as possible. To this end,the pole pieces themselves may be used to load a wave guide to formtherefrom a wave interaction circuit. Alternatively, the permanentmagnets may be used directly without the use of separate pole pieces.

What is claimed is:

1. In combination, means including an evacuated en-. velope and twoelectrodes spaced apart therein for defining a path ofelectron fio-w, asuccession of annular permeable members spaced apart along the path offlow for serving as a succession of polepieces, a succession of annulardisk permanent magnets positioned transverse to the path of flow andmagnetized in the radial direction, each of the successive magnetsassociated with one of the successive permeable members and adjacentmagnets magnetized in an opposite direction whereby successive permeablemembers are oppositely poled, and a permeable shielding membersurrounding the succession of permanent magnets for serving as thereturn path for the flux between adjacent magnets.

2. In combination, an evacuated envelope, an electron gun and targetspaced apart in said envelope for defining therebetween a path ofelectron flow, a succession of annular permeable members spaced apartalong the path of flow for serving as a succession of pole pieces, asuccession at annular disk magnets positioned transverse to the path offlow and magnetized in the radial direction, each magnet of thesuccession being positioned to have one 'pole face in contact with oneof the successive permeable members, the polarities of the pole faces incontact with adjacent members being opposite whereby successive membersare oppositely poled, and permeable shielding means enclosing themagnets for serving as the return path for the flux between adjacentmagnets whereby there results a succession of magnetic loops disposedalong the path of flow each having a gap corresponding to the spacebetween successive members adjacent the path of flow.

3. In a device which utilizes the interaction between an electromagneticwave and an electron beam, an envelope, an electron gun and targetspaced apart in said envelope for defining therebetween a path ofelectron flow, a slow wave transmission circuit positioned along thepath of flow for propagating -an electromagnetic wave in couplingrelation with said electron flow, and means for maintaining a highintensity magnetic field which is spatially alternating with a shortmagnetic period along the path of electron flow for focusing saidelectron flow,

said means including a succession of pole pieces spaced apart along thepath of flow, a succession of annular permanent magnets positionedtransverse of the path of flow and magnetized in the radial direction,each permanent magnet of the succession positioned to surround and have.with adjacent permeable members of the succession are oppositely poled,and permeable shielding means enclosing the magnets for serving as thereturn path for the flux between adjacent magnets,

4. In a traveling wave tube, an envelope, an electron gun and targetspaced apart in said envelope for defining therebetween a path ofelectron flow, a slow wave transmission circuit positioned along thepath of flow for propagating an electromagnetic wave in couplingproximity to said electron flow, and means for maintaining a largenumber of short high intensity magnetic field regions in successionalong the path of electron flow, the polarity of adjacent field regionsbeing reversed, said means including a succession of annular permeablemembers spaced apart along the path of flow for serving as a successionof pole pieces, a succession of annular disk magnets positionedtransverse to the path of flow and magnetized in the radial direction,each magnet of .the suc- 6 cession being positioned to have one poleface in contact with one of the successive permeable members, thepolarities of the pole faces of adjacent magnets in contact withadjacent members being opposite whereby successive members areoppositely poled, and permeable shielding means enclosing the magnetsfor serving as the return path for the flux between adjacent magnets.

References Cited in the file of this-patent UNITED STATES PATENTS2,300,052 Lindenblad Oct. 27, 1942 2,305,884 Litton Dec. 22, 19422,503,173 Reisner Apr. 4, 1950 2,741,718 Wang Apr. 10, 1956

